Transparent, conductive, metal oxide films, such as tin oxide films, on glass substrates are useful for various applications. When employed as electrodes for liquid crystal cells, such films must be in the form of a pattern to allow for digital or other configurational readout. The films are patterned conventionally by first applying a thin, uniform film of the metal oxide onto the substrate and then removing part of the film to form the desired pattern. This can be done by applying a photoresist, exposing the photoresist to light through a photomask, developing the photoresist, thereby uncovering that portion of the metal oxide film to be removed, etching away the uncovered portions of the metal oxide film and finally removing the remaining photoresist. It is obvious that this is a tedious and expensive process, although it is sensitive and very detailed and highly resolved patterns can be made in this manner.
Another requirement for liquid crystal cell electrodes is that the glass substrate be of high quality and contain very few alkali metal ions. Alkali metal ions in the glass migrate to the surface of the electrode and are a cause of misalignment of the liquid crystal cells, which in turn causes hazy, low contrast areas in the cell. This would preclude the use of inexpensive soft or soda-lime-silicate glasses for use in liquid crystal cells were it not for the discovery that the surface of these glasses can be depleted of alkali metal ions by exposing the glass to a glow discharge, as disclosed in copending application of Goodman et al., referred to hereinabove. This treatment removes alkali metal ions to a depth of several thousand Angstroms. Conductive metal oxide films deposited on such ion-depleted glass substrates have improved uniformity and quality as well.
In copending application of Carlson et al., "Deposition of Tin Oxide Films on Glass" filed concurrently herewith, Ser. No. 544,569, a process is disclosed whereby a transparent, conductive tin oxide coating is deposited and, at the same time, the area beneath the coating is depleted of alkali metal ions. This process comprises applying a D.C. glow discharge to the heated glass substrate in a vacuum chamber to which small amounts of a volatile tin compound and oxygen have been added under certain conditions. High quality, uniform tin oxide films can be made in this manner.
However, for use in liquid crystal cells, it would be highly desirable to be able to apply the metal oxide film on the glass substrate directly in the form of a pattern, as by covering the glass substrate to be coated with a suitable mask. When conventional insulating masking materials are used, however, such as fused silica or aluminum oxide masks, a net surface charge on the surface is acquired during glow discharge which interferes with the deposition of the tin oxide, resulting in poor edge definition of the pattern. Further, the area of the substrate beneath the masked areas are not ion depleted during the glow discharge, with the result that such areas still contain alkali metal ions which can contribute to misalignment of liquid crystal cells made from them. Thus, an improved method was sought whereby a metal oxide film could be deposited directly in the form of a pattern and the glass substrate depleted of alkali metal ions uniformly throughout its surface area.